This invention relates to a constant-current power supply and more particularly to an improvement in a constant current power supply to also serve as a constant voltage power supply.
A conventional three-terminal type constant voltage power supply is conventionally constructed utilizing voltage-dividing resistors across the load for negative feedback to a control circuit. This control circuit detects the amount of change in the output voltage due to a variation in the load and, in response thereto, controls an output transistor to maintain the output voltage constant.
A conventional three-terminal type constant current power supply is substantially similar to the above-described constant voltage power supply. However, a current-detecting element such as a resistor is used in series with the load for detection of the output current. The control circuit is responsive to the voltage developed across this resistor, for controlling the output transistor so as to insure a constant output current at all times, usually by comparing the voltage drop of this current-detecting resistor with a predetermined reference voltage.
A constant current power supply may also be constructed so that the control circuit controls an input, that is, a load current, to the output transistor so as to maintain a constant current output in response to a voltage level obtained from a current-detecting variable resistor at the power supply output.
In accordance with conventional practice, a constant voltage power supply and a constant current power supply of the type described must be separately provided in order to feed both a constant-current load and a constant-voltage load, and this has proven both expensive and inconvenient.